Use -Zinline-mir instead of -Zmir-opt-level=3

Except for mir inlining all mir opts that are beneficial for cg_clif now
run on -Zmir-opt-level=2. -Zmir-opt-level=3 enables some more expensive
optimizations.

Author: bjorn3

build_system/build_sysroot.rs

@@ -251,7 +251,10 @@ fn build_clif_sysroot_for_triple(
     rustflags
         .push_str(&format!(" --sysroot {}", RTSTARTUP_SYSROOT.to_path(dirs).to_str().unwrap()));
     if channel == "release" {
-        rustflags.push_str(" -Zmir-opt-level=3");
+        // Incremental compilation by default disables mir inlining. This leads to both a decent
+        // compile perf and a significant runtime perf regression. As such forcefully enable mir
+        // inlining.
+        rustflags.push_str(" -Zinline-mir");
     }
     compiler.rustflags += &rustflags;
     let mut build_cmd = STANDARD_LIBRARY.build(&compiler, dirs);